I want to write to a governmental organization to tell their users we have easier solution to their problem. Should I write a letter or still a proposal with executive summary?
Neither. They will not look at it. You need to find out who cares about the issue and get to them somehow. B2G is so very different, you telling them what the do not care about will not work. If you do not have a rapid supporter/sponsor in this chain, you will not make progress.
Ask for a meeting